The thorium cycle is a good candidate for replacing the current U/Pu cycle since the fissile nucleus of the cycle, 233U, has neutronic properties favourable to a much better regeneration of fissile material in thermal reactors. Moreover, the production of minor actinides is significantly reduced. However, the use of Thorium is only viable if the spent fuel is reprocessed to recover the fissile 233U that does not exist in nature. This reprocessing will involve a heavy industrial infrastructure, particularly since thorium based spent fuel contains small quantities of 232U that is the mother of the hard gamma emitter (208Tl) of 2.6 MeV. The goal of this thesis is, firstly, to study the parameters related to the synthesis of 232U in several kind of fuels and reactors. In a second part, the thesis focuses on the impact on radioprotection of the back end of the fuel in case of switching from the current uranium (U/Pu) cycle to the thorium (Th/U) cycle. For this last purpose, CHARS (CHAracterization of Radioactives Sources) was developed during this thesis. This code, validated by several benchmarks, handles the calculation of radiation sources in all aspects of the fuel cycle.
